<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Document</title>
</head>
<style>
  div{
    width: 500px;
    height: 500px;
    border: 3px solid blue;
    border-top:3px solid red;
    margin: 0 auto;
    font-size: 50px;
    text-align: center;
    line-height: 500px;
  }
</style>
<body>
  <div>
    一个盒子模型
  </div>
</body>
<script>

</script>
</html>